1. Field of the Invention
The present invention relates to a sheet feed cassette, and more particularly to a sheet feed cassette that is suited to be used in an image forming apparatus.
2. Description of Related Art
As an invention relating to a conventional sheet feed cassette, for example, an automatic sheet feeder disclosed by Japanese Patent Laid-Open Publication No. H11-116073 is known. The automatic sheet feeder comprises a stacker plate, an auxiliary press-up plate, a first coil spring, a second coil spring and a feed roller. A plurality of sheets are stacked on the stacker plate. The first coil spring presses up the stacker plate, whereby the leading edge of the stacked sheets is pressed against the feed roller. The auxiliary press-up plate is located below the stacker plate, and the second coil spring presses up the auxiliary press-up plate, whereby the stacker plate is pressed up.
In the automatic sheet feeder, when sheets to be fed are long in a sheet feeding direction, the auxiliary press-up plate presses up the stacker plate. On the other hand, when sheets to be fed are short in the sheet feeding direction, the auxiliary press-up plate does not come in contact with the stacker plate. Therefore, when sheets to be fed are long and accordingly heavy, the sheets are lifted up by a relatively great force, and when sheets to be fed are short and accordingly light, the sheets are lifted up by a relatively small force. Consequently, the force to press the leading edge of the stacked sheets against the feed roller is almost constant regardless of the size of the stacked sheets. Thus, in the automatic sheet feeder disclosed by Japanese Patent Laid-Open Publication No. H11-116073, sheet feeding errors can be reduced.
In the automatic sheet feeder disclosed by Japanese Patent Laid-Open Publication No. H11-116073, however, in stacking sheets on the stacker plate, a great force is necessary to press down the stacker plate. More specifically, in stacking a large number of sheets in the automatic sheet feeder, a user needs to set the sheets on the stacker plate and thereafter press down the stacker plate. In this way, the sheets are loaded in the automatic sheet feeder. However, when heavy sheets are set, the stacker plate is pressed up by the auxiliary press-up plate, and in this case, the user needs to press down the stacker plate against the elastic force of the first coil and the elastic force of the second coil. Therefore, in the automatic sheet feeder disclosed by Japanese Patent Laid-Open Publication No. H11-116073, in stacking sheets on the stacker plate, a user needs to apply a great force to press down the stacker plate.